One problem encountered in cameras, and in particular in the case of cameras having electrically driven advance and rewind systems, is that of decoupling the shutter system during rewind so that accidental depression of the shutter button does not cause repeated tripping of the shutter, resulting in double exposure of the film. This problem can be particularly acute in high speed electric rewind systems, where a few seconds' inattention on the part of the operator can cause loss of film footage. The origin of this problem lies in the fact that the shutter cocking system is generally directly actuated by rotation of a film engaging sprocket used to govern the length of each one-frame film advance. Since the sprocket must of necessity rotate during rewind as well as during advance, a great many camera designs suffer from the limitation that rotation of the sprocket in the rewind direction will cock the shutter mechanism as readily as it does when driven in the film advancing direction. Such bidirectonal shutter cocking gives rise to the abovementioned problem of accidental multiple film exposure during rewind if the shutter button is held depressed during rewind. A simple and inexpensive solution to this problem would clearly be a useful feature for such cameras.
Another problem frequently encountered in electrically driven cameras arises from the fact that during film advance the shutter cocking system places an unduly varying time-changing load on the motor drive system, which causes an undesired noisy camera operation and undesired heavy peak load on the drive mechanism of the camera. This arises principally because of the fact that during shutter cocking, a sprocket driven cocking lever or member is driven from a relaxed to a shutter cocking position against the force of an energizing spring associated therewith. Since the restoring force of the spring increases with its extension, the motor load will normally rise during that fraction of the film advance operation in which the cocking lever is moved between these two positions. A consequence of this time-varying load on the motor system is that the motor and its associated gearing must be chosen so as to overcome the peak torque reflected through the gear train by the cocking load, and yet provide sufficient average drive speed to advance one frame in a reasonable time, e.g., one-quarter to one-half second. The result is that after the cocking lever has been cocked and its load thereafter removed from the motor drive system, the drive system rapidly speeds up, since the cocking load is typically the major load thereon. This final film advancing speed is much higher than the average speed necessary to advance the film and causes a noisy high speed gear whine. Suppression of such noise by appropriate modification of the film advancing and shutter cocking system is also a needed improvement in motor driven cameras.
Moreover, at the end of film advance, the motor drive system is normally brought to a sudden stop by throwing a short circuit across the motor winding, thereby dissipating the angular kinetic energy of all rotating systems, in particular that of the motor armature. Since kinetic energy varies as the square of the angular velocity of the armature, and since the total frame advance time is governed solely by the average value of the angular velocity, it follows that such overspeed terminal drive rate represents wasted battery energy. It would thus be a desirable feature to eliminate terminal overspeed for energy reasons as well.